1. Field of the Invention
The present invention pertains generally to the field of power transistors and, more particularly, to methods and apparatus for providing a constant gain, constant phase RF power transistor block for use in, e.g., a base station receivers in a high frequency wireless communication networks.
2. Background
The use of radio frequency (RF) power transistors, for example, as amplifiers in wireless communication networks, is well known. With the considerable recent growth in the demand for wireless services, such as personal communication services (PCS), the operating frequency of wireless networks has increased dramatically and is now well into the gigahertz (GHz). At such high frequencies, consistent performance characteristics of RF power transistor amplifier devices becomes increasingly more complex.
In particular, RF amplifier requirements demand highly linear performance to meet the demands of new digital systems, such as CDMA, HDTV, WCDMA, GSM, etc. Cellular systems are especially demanding, because the power transistor devices are expected to maintain the same output characteristics, even if the main power fails and back-up DC battery kicks in. This is problematic, since the voltage supplied by such back-up DC battery systems can vary widely.
Production of RF power transistors on a large scale basis is also a problem, because of natural variables which the devices possess. In particular, the transistor devices have natural variances in output gain and signal phase shift, especially over varying input voltages. Thus, in commercial implementations, significant time and effort is needed to first characterize each RF transistor device over the range of expected operating voltages, and then attempt to xe2x80x9ctunexe2x80x9d the device to deliver a desired output gain and phase. However, the ability to successfully tune transistor devices is limited, due to the fact that the transistors will often exhibit differing gain and/or phase variations over identical operating voltage changes.
One known technique is to match the transistor output to a higher impedance, e.g., 50 ohms, which greatly reduces the possibility of variations in gain or phase caused by being placed in different amplifier circuit configurations. While this technique can greatly reduce problems in proving constant performance gain and phase delay output across different transistor devices, it does not completely solve the problem. This is because all RF transistors, if taken in a large enough sample size, will have gain and phase delay variations for any specific voltage. This variation can make large scale production of advanced RF amplifiers extremely time consuming and costly. Further, if the operating voltage provided to the amplifier changes, such as when a base station loses main power and goes on battery back-up, the gain and phase delay changes in the devices will not be known, without time consuming and expensive individual xe2x80x9ctry and failxe2x80x9d tuning.
By way of illustration, even if two RF transistor devices could be xe2x80x9ctunedxe2x80x9d in respective identical amplifier circuits to have output gain of 11 dB gain, 6 degrees phase change at an input voltage of 28 v dc, a drop in the input voltage to 27.5 v dc would likely cause different results in the respective RF transistors, e.g., with the output of one changing to 10.9 dB and 6 degrees phase delay, and the other changing to 10.7 dB and 5.8 degrees phase delay. This difference in operating characteristics between transistor devices is problematic for the manufactures and users of the RF transistors.
Thus, it would be advantageous to provide RF power transistor amplifier devices that have a constant gain and phase delay output over a wide variation in input voltages.
In accordance with a first aspect, the present invention provides a constant gain, constant phase RF power block, e.g., for use in a RF amplifier apparatus. In a preferred embodiment, the power block includes a DC to DC power supply circuit having as an input a varying DC voltage and as outputs a constant supply voltage and a constant bias voltage. A power transistor circuit is provided adjacent the power supply circuit, the power transistor circuit having as inputs the constant supply voltage and the constant bias voltage, and further configured to receive and amplify an RF signal.
In accordance with a further aspect of the invention, the power supply circuit includes means for setting the supply and bias voltage levels. In one preferred embodiment, a first laser trimmable resistor is provided for setting the supply voltage and a second laser trimmable resistor is provided for setting the bias voltage. In this manner, the constant supply and bias voltages may be easily tuned to a desired level during assembly of the power block device. In an alternate preferred embodiment, the supply and bias voltages are set with respective potentiometers.
In accordance with a still further aspect of the present invention, the power supply circuit may comprise either a sepic converter (i.e., which either steps up or steps down the input voltage) or a forward converter (i.e., which steps down the input voltage).
In accordance with yet another aspect of the present invention, the input and amplified RF signals are each matched to a relatively high impedance, e.g., approximately fifty ohms in a preferred embodiment.
In another preferred embodiment, the present invention provides an amplifier apparatus, comprising a heat sink. A DC to DC power supply circuit having as an input a varying DC voltage and as outputs a constant supply voltage and a constant bias voltage, includes a transformer housing secured to the heat sink. A power transistor device having as inputs the constant supply voltage and the constant bias voltage, includes a mounting flange secured to the heat sink. In accordance with a still further aspect of the present invention, the transformer housing and flange are attached to the heat sink proximate each other.
As will be apparent to those skilled in the art, other and further aspects and advantages of the present invention will appear hereinafter.